Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With testing – sampling or analyzing
Reexamination Certificate
1997-09-19
2001-01-16
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With testing, sampling or analyzing
C162S062000, C162S065000, C162S089000, C700S117000, C700S266000
Reexamination Certificate
active
06174409
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related in general to bleaching of cellulose pulps. More specifically, the present invention concerns methods of bleaching cellulose pulps with chlorine dioxide.
2. Related Art
Bleaching of cellulose pulps typically employs chlorine containing chemicals. Efforts have been made to reduce their use because of pollution concerns, however their use has continued. This due at least in part because the brightness of the paper made from the pulps obtained using other oxidizers is either obtained at greater cost, and/or the strength of the paper ultimately produced from the bleached cellulose pulp does not meet manufacturer and consumer requirements.
Some pulps processors using chlorine dioxide will use the chlorine dioxide to bleach cellulose pulp in the initial pulp bleaching stage or stages, and then, in additional stages, add a non-chlorinated oxidizer for meeting final brightness specifications of the paper. This approach, while commendable because the final strength of the paper is good, leads to only a small reduction in actual chlorine dioxide usage. Other processors have added chlorine dioxide and non-chlorine-containing oxidizers to pulp to be bleached in one and the same stage; however, these pulp processors have not adjusted the ratio of chlorine dioxide to non-chlorine-containing oxidizer in a manner that affects strength of the final paper produced. Indeed such a relationship of this ratio to paper strength has heretofore not been recognized by prior practitioners. The only direction given on how to achieve good strength characteristics of the final paper when using ozone and chlorine dioxide is the viscosity of the treated pulp. However, a drawback of the viscosity test as a selectivity rating is that it is known that the correlation between pulp viscosity and strength characteristics of ozone-treated pulps differs from that of chlorine-treated pulps. (It can also be seen in Example-2/Table 2 &
FIGS. 3 & 4
) Depending on the reaction conditions and the treatment history of the pulp, ozone-treated pulps can have much lower viscosity values than pulps bleached with chlorine-based sequences to the same strength (see
Pulp Bleaching: Principles and Practice
, by C. Dence and D. Reeve, p. 734, published 1996 by TAPPI Press).
There is thus a genuine need in the art of cellulose pulp bleaching for methods of bleaching these pulps using chlorine dioxide and ozone in ratios that do not sacrifice pulp brightness or strength characteristics of the final paper made from the bleached pulps, while also keeping in mind the strict environmental rules and regulations now in force in many regions of the world.
SUMMARY OF THE INVENTION
In accordance with the present invention, methods are presented which surprisingly recognize and overcome some if not all of the problems encountered in the prior art. The inventive methods are valid for conventional cooked and oxygen delignified pulps.
One aspect of the invention is a method of bleaching cellulose pulp using chlorine dioxide and ozone in one and the same first stage of a bleaching sequence having a plurality of stages, the method comprising controlling a ratio of chlorine dioxide to ozone in a first stage of a first bleaching sequence in a range effective to achieve strength characteristics of a final paper elaborated from the first bleaching sequence at least as great as strength characteristics when compared to a control bleaching sequence, the control bleaching sequence being the same in all respects as the first bleaching sequence except not employing ozone in the first stage, and for essentially the same final brightness of pulp elaborated from the first and the control bleaching sequence.
A second aspect of the invention is a method of bleaching cellulose pulp in a bleaching sequence (D/Z)E
op
PD or (Z/D)E
op
PD to produce a novel bleached pulp, the method comprising employing a ratio of chlorine dioxide to ozone in a first bleaching stage (D/Z) or (Z/D), the ratio effective to produce a first set of curves of properties defining strength characteristics of the novel bleached pulp to be substantially similar to or better than a second set of curves of properties defining strength characteristics of the same beginning pulp bleached using a control bleaching sequence DE
op
D to produce a control bleached pulp, the novel bleached pulp and the control bleached pulp having essentially the same brightness.
A third aspect of the invention is a method of bleaching cellulose pulp in a bleaching sequence (D/Z)E
op
DE
p
D or (Z/D)E
op
DE
p
D to produce a novel bleached pulp, the method comprising employing a ratio of chlorine dioxide to ozone in a first bleaching stage (D/Z) or (Z/D), the ratio effective to produce a first set of curves of properties defining strength characteristics of the novel bleached pulp to be substantially similar to or better than a second set of curves of properties defining strength characteristics of the same beginning pulp bleached using a control bleaching sequence DE
op
DE
p
D to produce a control bleached pulp, the novel bleached pulp and the control bleached pulp having essentially the same brightness.
A fourth aspect of the invention is a method of bleaching cellulose pulp in a bleaching sequence O(D/Z)E
op
D or O(Z/D)E
op
D to produce a novel bleached pulp, the method comprising employing a ratio of chlorine dioxide to ozone in a first bleaching stage (D/Z) or (Z/D), the ratio effective to produce a first set of curves of properties defining strength characteristics of the novel bleached pulp to be substantially similar to or better than a second set of curves of properties defining strength characteristics of the same beginning pulp bleached using a control bleaching sequence ODE
op
D to produce a control bleached pulp, the novel bleached pulp and the control bleached pulp having essentially the same brightness.
A fifth aspect of the invention is a method of bleaching cellulose pulp in a bleaching sequence O(D/Z)E
o
D or O(Z/D)E
o
D to produce a novel bleached pulp, the method comprising employing a ratio of chlorine dioxide to ozone in a first bleaching stage (D/Z) or (Z/D), the ratio effective to produce a first set of curves of properties defining strength characteristics of the novel bleached pulp to be substantially similar to or better than a second set of curves of properties defining strength characteristics of the same beginning pulp bleached using a control bleaching sequence ODE
o
D to produce a control bleached pulp, the novel bleached pulp and the control bleached pulp having essentially the same brightness.
Pulps and final paper products made using one of the methods of the invention are also considered within the invention.
As used herein the terms used to describe the invention have the following meanings:
“brightness” means the value of ISO brightness using the ISO 3688-1977 method for handsheet preparation and brightness measurement, which is incorporated herein by reference, and “essentially the same brightness” means brightness measurements being the same within standard analytical chemistry laboratory error calculations;
“ratio” means the mass of chlorine dioxide divided by the mass of ozone, both relative to one ton of oven dried pulp used in the first stage of the inventive bleaching sequences;
“stage” has the meaning of TAPPI standard number TIS 0606-21, dated 1988, incorporated herein by reference;
the letter designations D, E, Z, Eo, Eop, having meaning derived from TAPPI standard number TIS-0606-21, dated 1988, which is incorporated herein by reference;
the designation “(D/Z)” means a single bleaching stage wherein chlorine dioxide and ozone are introduced to the pulp either at the same point, or at points separated from each other, or in mixed form, with the proviso that (D/Z) means chlorine dioxide is introduced first in the stage;
the designation “(Z/D)” means a single bleaching stage wherein chlorine dioxide and ozone are introduced to the pulp either at the same point, or at points separated from e
Corbeil Sebastien
Fisher Steven A.
Sundaram V. S. Meenakshi
Alvo Steve
American Air Liquide Inc.
Burns Doane Swecker & Mathis L.L.P.
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